Talanta
Talanta 45 (1997) 237 248
ELSEVIER
Principles and analytical applications of acousto-optic tunable
filters, an overview
Chieu D. Tran
Department qf Chemistry, Marquette Universio,, P.O. Box 1881, Milwaukee, WI 53201-188L USA
Received 22 November 1996; accepted 19 February 1997
Abstract
Advantages of acousto-optic tunable filters have been exploited to develop novel analytical instruments which are
not feasible otherwise. The instrumentation development and unique features of such AOTF based instruments
including the multidimensional fluorimeter, the multiwavelength thermal lens spectrometer, the near-infrared spectrometer based on erbium doped fiber amplifier (EDFA), and detectors for high performance liquid chromatography
(HPLC) and flow injection analysis (FIA), will be described. © 1997 Elsevier Science B.V.
1. Introduction
Acousto-optic tunable filter (AOTF) is an allsolid state, electronic dispersive device which is
based on the diffraction of light in a crystal [1-9].
Light is diffracted by an acoustic wave because
when an acoustic wave propagates in a transparent material, it produces a periodic modulation of
the index of refraction (via the elasto-optical effect). This, in turn, will create a moving grating
which diffracts portions of an incident light beam.
The diffraction process can, therefore, be considered as a transfer of energy and momentum.
However, conservation of the energy and momentum must be maintained. The equation for conservation of m o m e n t u m can be written as [1]:
ka
=
ki + k~
(1)
where ki, ka and k, are the wave vector of the
incident and diffracted light, and of the phonon.
In the case of the AOTF, the acousto-optic
interaction occurs in an anisotropic medium, and
the polarization of the diffracted beam is orthogonal to that of the incident beam. The m o m e n t a of
incident and diffracted photons are:
Ikdl =
27rnd/2
(3)
They are not equal since one is ordinary ray and
the other is extraordinary ray (i.e. n~ # na).
In the case of collinear A O T F , the incident and
diffracted light beams, and the acoustic beam are
all collinear. If the incident light is an extraordinary and the diffracted light is an ordinary ray,
the m o m e n t u m matching condition becomes:
kd = hi -- ks
£
0039-9140/97..'$17.00 © 1997 Elsevier Science B.V. All rights reserved.
PII SO0 39-91 40(97)00146-X
=
vs(n~ -- no)/2
(4)
(5)
238
C.D. Tran / Talanta 45 (1997) 237 248
where Ik=[= 21rfs/V S and f~ and vs are the frequency
and velocity of the acoustic wave. Eq. (5) can be
generalized for all types of AOTFs including the
collinear and the noncollinear AOTFs:
j~ =
v~(n~ -- no)(sin 4 Oi + sin 2 20i) I/2
2
(6)
where 0i is the incident angle. When 0~ = 90 ° Eq.
(6) reduced to Eq. (5), i.e. the case of collinear.
It is thus evidently clear that in an anisotropic
crystal, where the phase matching requirement is
satisfied, diffraction occurs only under optimal
conditions. These conditions are defined by the
frequency of the acoustic waves and the wavelength of a particular diffracted light. For a given
acoustic frequency, only light whose wavelength
satisfies either Eq. (5) or Eq. (6) is diffracted from
the crystal. The filter can therefore, be spectrally
tuned by changing the frequency of the acoustic
waves (i.e. J~).
Generally, the AOTF is fabricated from an
anisotropic TeO2 crystal and onto it an array of
LiNbO3 piezoelectric transducers are bonded. A
radio frequency (RF) signal is applied to the
transducers which, in turn, generates an acoustic
wave propagating through the TeO2 crystal. These
propagating acoustic waves produce a periodic
moving grating which will diffract portions of an
incident light beam. A light beam propagating as
an e-ray is converted into an o-ray and in addition, is spatially separated from the original e-ray
by interaction with, and diffraction from, an
acoustic wave propagating in the same medium
[1 9]. As illustrated in Eq. (6), for a fixed acoustic
frequency and sufficiently long interaction length,
only a very narrow band of optical frequencies
can approximately satisfy the phase matching
condition and be diffracted. The wavelength of
the diffracted light can therefore, be tuned over
large spectral regions by simply changing the frequency of the applied RF signal. The scanning
speed of the filter is defined by the speed of the
acoustic wave in the crystal, which is on the order
of microseconds (~ts). As a consequence, compared to conventional gratings, the AOTFs offer
such advantages as being all-solid state (contains
no moving parts), having rapid scanning ability
(las), wide spectral tuning range and high through-
put, allowing high speed random or sequential
wavelength access, and giving high resolution (a
few angstroms). The AOTF has offered unique
means to develop novel instruments which are not
possible otherwise. The instrumentation development and unique features of such AOTF based
instruments including the multidimensional
fluorimeter, the multiwavelength thermal lens
spectrometer, the near infra-red spectrometer
based on erbium doped fiber amplifier (EDFA),
and detectors for high performance liquid chromatography (HPLC) and flow injection analysis
(FIA), will be highlighted in this overview.
2. Applications in spectroscopy
2.1. A O T F - b a s e d f l u o r i m e t e r
Fluorescence technique has been demonstrated
to be a sensitive method for trace characterization. Since realtime samples are generally mixtures
of many different compounds, their analyses usually require measurements of the fluorescence
spectra at different excitation wavelengths (because absorption spectra of compounds in the
mixture are different). Fluorimeter based on multichannel detection (e.g. videofluorimeter) has
been developed to alleviate this time consuming
process [10,11]. While the instrument has proven
to be a very powerful method for the determination of multicomponent trace chemical species, its
applications are still limited because of such factors as high cost, low sensitivity, slow in the data
acquisition (the fastest is on the order of milliseconds) and complicated data analysis [10,11].
These limitations can be eliminated if the AOTF
is used to develop a new generation multidimension spectrofluorimeter.
As explained above, an incident white light is
diffracted by the AOTF into a specific wavelength
when a specific RF is applied to it. It is important
it realize that the diffracted light needs not be a
monochromatic light. Multiwavelength light can
be diffracted from the AOTF when more than
one RF signals are simultaneously applied into
the filter [1 9]. As a consequence, the AOTF can
be used as a polychromator. Compared to con-
C.D. Tran
Talanta 45 (1997) 237 248
ventional polychromators, advantages of this electronic AOTF polychromator include its ability to
individually amplitude-modulate each wavelength
of the diffracted multiwavelength light at different
frequency. This is accomplished by individually
and sinusoidally modulating each applied RF signal at a different frequency. This feature together
with the fast scanning ability make the AOTF to
be uniquely suited for the development of a novel,
all solid-state, nonmoving parts multidimensional
spectrofluorometer.
The schematic diagram of the AOTF based
multidimensional fluorimeter is shown in Fig. l
[2,3]. Two AOTF's were used in this instrument:
one for excitation and the other for emission. The
first AOTF was used to specifically diffract white
incident light into a specific wavelength(s) for
excitation. Depending on the needs, the second
AOTF (i.e. the emission AOTF) can be used as
either a very fast dispersive device or a polychromator. In the first configuration (i.e. the rapid
scanning fluorimeter), the sample was excited by a
single excitation wavelength; the emitted light was
analyzed by the emission AOTF which was
scanned very fast. A speed of 4,8 A las ~ was
found to be the fastest speed which the AOTF can
be scanned with a reasonable S/N and resolution.
With this speed, a spectrum of 150 nm can be
measured in 312 ~ts. Faster scanning is possible,
but because of the limitation due to the speed of
the acoustic wave, may undesiredly lead to the
degradation in the S/N and spectral resolution
[2,3]. In the second configuration (i.e. multidimenS
computer
~
fromcomputer
Fig. 1. Schematic diagram of the AOTF based fluorimeter:
PD, photodiode; S, sample; P, polarizer; PMT, photomultiplier tube; amp, amplifier; osc, oscillator; mod, modulator;
comb, combiner.
239
sional fluorimeter), both AOTFs were used as a
polychromator. Several different RF signals were
simultaneously applied into the first AOTF to
provide multiple excitation wavelengths. The
emission was simultaneously analyzed at several
wavelengths by the emission AOTF, With this
configuration, the fluorimeter can be used for the
analysis of multicomponent samples, and the
maximum number of components it can analyze
is, in principle, a x b where a and b are the
number of excitation and emission wavelengths,
respectively [3]. In fact, multicomponent samples,
e.g. mixtures of rhodamine-B, fluorescein, eosin
and 4-(dicyanomethylene)-2-methyl-6-[p(dimethylamino)styryl]-4-H-pyran (DMP) can be simultaneously analyzed at a limit of detection of 10 ~0
M [2,31.
2.2. A OTF-based thermal lens spectrophotometer
The fluorescence technique is very sensitive and
can be used for the determination of trace chemical species at very low concentration. However,
the technique is not applicable to all compounds
because only few molecules are fluorescent. Therefore, it is important that a novel technique which
has the same sensitivity as the fluorescence technique, but is applicable to non-fluorescent compounds be developed. Thermal lens technique is
one of such possibilities.
The thermal lens technique is based on the
measurement of the temperature rise that is produced in an illuminated sample by nonradiative
relaxation of the energy absorbed from a laser
[12-17]. Because the absorbed energy is directly
measured in this case, the sensitivity of the technique is similar to that of the fluorescence technique, and is relatively higher than the
conventional absorption measurements. In fact it
has been calculated and experimentally verified
that the sensitivity of the thermal lens technique is
237 times higher than that by conventional absorption techniques, when a laser of only 50 mW
power was used for excitation [12 17]. Absorptivities as low as 10- 7 have been measured using this
ultrasensitive technique [12 17]. Potentially, the
technique should serve as an excellent method for
trace chemical analysis because it has high sensi-
240
C.D. Tran / Talanta 45 (1997) 237 248
Fig. 2. Schematicdiagram of the multiwavelengththermal lens
spectrometer based on an AOTF as a polychromator: amp,
RF power amplifier; osc. oscillator; mod, sinusoidal modulator; PIN, photodiode; F, interferencefilter; Ph, pinhole.
tivity, in situ and non-destructive ability, and
requires a minimum amount of sample. Unfortunately, its applications to the area of general trace
chemical analysis are not so widespread in comparison to other spectrochemical methods. A variety of reasons might account for its limited use,
but the most likely one is probably due to the low
selectivity. The majority of reported thermal lens
spectrometers employ only a single excitation
wavelength [12-17], and as a consequence can
only be used for the analysis of one component
samples. A multiwavelength thermal lens spectrometer is needed to analyze, in realtime, multicomponent samples without any pretreatment.
A O T F offers a unique means for the development
of the first, all solid state, no-moving part fast
scanning multiwavelength thermal lens instrument.
The schematic diagram of the A O T F based
multiwavelength thermal lens spectrometer is
shown in Fig. 2 [4,18]. This instrument is based on
the use of the A O T F as a polychromator. An
argon ion laser operated in the multiline mode
was used as the light source. This multiwavelength
laser beam was converted into the excitation beam
with appropriate number of wavelengths by
means of a TeO2 AOTF. In this cased, four
different wavelengths were used. They were obtained by simultaneously applying four different
RF signals to the AOTF. These RF signals were
provided by four different oscillators (oscl, osc2,
osc3 and osc4). To differentiate each wavelength
from the others, the applied RF's were sinusoidally amplitude modulated at four different
frequencies by four modulators (modl, rood2,
rood3 and mod4). The four AM modulated RF
signals were then combined by means of a combiner, and amplified by a RF power amplifier
(amp). The amplified, AM modulated signal was
then applied onto the A O T F to enable it to
diffract the incident multiline laser beam into
beam which has the four different wavelengths.
The probe beam, provided by a He-Ne laser, was
aligned to overlap with the pump beam at the
sample cell by means of a dichroic filter (DF). The
heat generated by the sample absorption of the
pump beams changes the intensity of the probe
beam. The intensity fluctuation of the probe beam
was measured by a photodiode (PD2) placed behind a 632.8 nm interference filter (F) and a slit
(S). A lens was used to focus the probe beam, and
the distance between this lens and the sample was
adjusted to give maximum thermal lens signals.
The signal intensity, measured as the relative
change in the probe beam center intensity, was
recorded by a microcomputer through an AD
interface board [4,18].
Compared with other multiwavelength thermal
lens instruments~ this all solid state thermal lens
spectrophotometer has advantages which include
its ability to simultaneously analyze multicomponent samples in microsecond time scale, without
the need for any prior sample preparation. In fact,
with this apparatus and with the use of only 12
mW multiwavelength excitation beam, multicomponent samples including mixtures of lanthanide
ions (Er 3+ , Nd 3 ~, Pr 3 + and Sm 3 +) can be simultaneously determined with a LOD of 10 6 cm 1
[18].
2.3. Near infra-red spectrophotometer based on
the A O T F and an erbium doped fiber amplifier
The use of the near infra-red spectrometry in
chemical analysis has increased significantly in the
last few years [15-17,19-25]. The popularity
stems from the advantages of the technique,
namely its wide applicability, noninvasive and
on-line characteristics. Near infra-red region cov-
C.D. Tran
Talanta 45 (1997) 237 248
ers the overtone and combination transitions of
the C - H , O - H and N - H groups, and since all
organic compounds possess at least one or more
of these groups, the technique is applicable to all
compounds [19 25]. There is no need for pretreatment of the sample, and since NIR radiation
can penetrate a variety of samples, the technique
is noninvasive and has proven to be potentially
useful for noninvasive, on-line measurements. In
fact, NIR technique has been used in industries
for on-line measurements for quality control and
assurance. However, its applications are not as
widely as expected. This may be due to a variety
of reasons but the most likely ones are probably
due to the limitations on the speed, stability and
light throughput of the currently available instruments. To be effectively used as a detector for
on-line measurements, the NIR instrument needs
to have high and stable light throughput, to suffer
no drift in the baseline, and can be rapidly
scanned. These impose severe limitations on conventional NIR spectrophotometers because such
instruments generally have relatively low and unstable light throughput, suffer some degree of
baseline drift, and can only be scanned very
slowly. AOTF, with its ability to rapidly scan as
well as to control and maintain the intensity of
light at a constant level, offers a means to alleviate some of these limitations.
It has been shown recently that stimulated
emission can be achieved in a fiber when the fiber
is doped with rare earth ions such as Er 3 +, and
optically pumped by an ion laser or YAG laser
[26,27]. Now, for the first time, a lasing medium
can be confined in a material as flexible and as
small ( < 10 ~tm) as a single mode fiber [19,20].
Because of such features, the length of the doped
fiber can be adjusted to as long as few kilometers
to enable the fiber to have optical output in the
range of kilowatts. Furthermore, the doped fiber
can be pumped by a high power diode laser
fusion-spliced directly into the doped fiber. It is
thus, evidently clear that this all-fiber, compact
E D F A can provide near infra-red light with
highest intensity and widest spectral bandwidth
compared to other (cw) near-IR light sources
currently available. A novel, compact, all solid
state, fast scanning near infra-red spectrophoto-
241
meter which has no moving part, high and very
stable light throughput can, therefore, be developed by use of this E D F A as a light source and
AOTF as a dispersive element to scan and control
the intensity of the diffracted light. Schematic
diagram of such a spectrophotometer is shown in
Fig. 3 (with the insert showing the construction of
the EDFA). As illustrated, the near infra-red light
from the output of the E D F A was dispersed to
monochromatic light and spectrally scanned by an
AOTF. The RF signal, provided by a driver, was
amplitude-modulated at 50 kHz by a home-built
modulator and amplified by a RF power amplifier
prior to being applied to the AOTF. The light
diffracted from the A O T F was split into two
beams (i.e. sample and reference beams) by means
of a beamsplitter (BS). Intensity of the light in the
sample and reference beams was detected by
thermo-electrically cooled InGaAs detectors. The
output signals from the (reference and sample)
detectors which were amplitude modulated at 50
kHz were connected to lock-in amplifiers (Stanford Research Systems Model SR 810) for demodulating and amplifying. The signal from the
reference beam can be used either as a reference
signal for a double beam spectrophotometer or as
a reference signal for the fed-back loop to stabilize the intensity of the light in the sample beam in
a manner similar to those used previously [1,4].
The signals from the lock-in amplifiers were then
connected to a microcomputer through a 16 bit
A/D interface board.
As expected, the sensitivity of this EDFAAOTF based spectrophotometer is comparable
with those of the halogen tungsten lamp-AOTF
based instruments. In fact, this spectrophotometer
can be used to detect water in ethanol at a limit of
detection of 10 ppm. More importantly is its high
light throughput. The intensity of this E D F A
light source was found to be about 20 times
higher than that of 250 W halogen tungsten lamp.
As a consequence, it can be used for measurements which are not possible with lamp based
instruments. Two measurements were performed
to demonstrate this advantage. Shown in Fig. 4 is
the intensity of the light transmitted through six
sheets of photocopy paper (Cascade X-9000 white
paper). As illustrated, because of the high absorp-
C.D. Tran / Tolanta 45 (1997) 237-248
242
Er doped~ber
/
980 nm LD
I=¢~ator
I=olatot
m out
\Bs
JRFam01
~RFgen I
F "7 reference
I InGaAs
I
computer
..........................
, ...........
.
Fig. 3. Schematic diagram of the near infra-red spectrophotometer based on the use of the EDFA as a light source and AOTF as
a dispersive element: EDFA, erbium doped fiber amplifier; P, polarizer; AOTF, acousto-optic tunable filter; PH, pinhole; BS,
beamsplitter; L, lens; RF amp, RF power amplifier; RF gen, RF generator; lnGaAs, detector. Insert: Schematic diagram of the
erbium-doped fiber amplifier system: LD, laser diode; WDM, wavelength division multiplexer.
tion (of the papers) and low intensity (of the
halogen tungsten lamp) no light was transmitted
when the halogen tungsten lamp based spectrophotometer was used. Because of its high intensity, a substantial
amount
light was
transmitted through the papers. In the second
measurement which is shown in Fig. 5, it was not
possible to use a lamp based spectrometer to
measure absorption spectrum of 1.0 M solution of
Pr 3 + in D 2 0 (in a 2 mm cell) placed after four
sheets of photocopy paper (dashed line). Because
of its high intensity, substantial amount of E D F A
C.D. Tran "Talanta 45 (1997) 237 248
200
243
3. Applications in chemical separations
---
i
3.1. A O T F based detector j o r H P L C
lO0
1500
1550
1600
Wavelength (rim)
Fig. 4. Relative intensity of the light transmitted through six
sheets of paper, measured with a 250 W halogen tungsten
lamp based spectrophotometer (
), and with the EDFA
based spectrophotometer(-- ).
light was transmitted through the papers and
Pr 3+ solution, and as a consequence, absorption
spectrum of the latter can be recorded (Fig. 5,
insert). This high throughput advantage is of particular importance in NIR measurements because
NIR techniques are often used for measurements
in which the signal of interested is very small and
riding on top of a very large background signal.
As such, it is very difficult, inaccurate and sometimes not possible to perform such measurements
with lowlight-throughput spectrophotometers.
15o [ - -
i ~0 [
1
!
-
::
"
....
1530
1540
~.
]
\
A , 0,,1
/2
1520
I,
221:
1550
.
1560
1570
Wavelength(nm)
Fig. 5. Relative intensity of the light transmitted through four
sheets of paper and 1.0 M solution of P r 3 + in D20 (in a 2 mm
pathlength cell), measured with a 250 W halogen tungsten
lamp based spectrophotometer (
), and with the EDFA
based spectrophotometer ( - - ) . Insert: spectra of the same
sample plotted as absorption spectra.
High performance liquid chromatography
(HPLC) has increasingly become the technique of
choice for chemical separations. As the technique
becomes more prevalent the demand for detectors
that can provide quantitative as well as qualitative
information on the analyse increases.
Variable wavelength absorption detectors are
the most widely used detectors for HPLC. However, this type of detector can only be used as a
quantitative technique because the qualitative information obtained from this detector is rather
limited, namely, it relies on the use of the retention time as the only tool for identification. The
development of diode array detectors (DADs) in
the early 1980s made it possible to obtain information on peak purity and identity [28]. Specifically, with a DAD based detector, the spectrum
obtained for each peak in the chromatogram can
be stored, and the subsequent comparison with
standard spectra will facilitate the identification of
peaks [28]. The optimum wavelength for single
wavelength detection can easily be found [28].
Wavelength changes can be programmed to occur
at different points in the chromatogram, either to
provide maximum sensitivity for peaks, or to edit
out unwanted peaks, or both [28]. Unfortunately,
in spite of their advantages, DADs still suffer
from limitations including their relatively high
cost and their low sensitivity (compared to variable and fixed wavelength detectors). It is therefore, of particular importance that a novel
detector which has higher sensitivity, low cost and
possesses all of the DADs' advantages be developed. The A O T F with its unique features is particularly suited for the development of such a
detector. Specifically, with its microsecond scanning speed, the A O T F based detector can rapidly
record absorption spectrum of a compound as it
elutes from the column. The random access to
wavelength(s) makes it possible to change and/or
to program the detector to any wavelength(s) to
obtain optimal detection. However, different from
the DADs, the AOTF-based detector is a single
channel detection technique, i.e. it is based on a
244
C.D. Tran / Talanta 45 (1997) 237 248
slit
slit
AOTF
Z
Flowcell
-
Xenon I Lens
•
lamp
Fig. 6. Schematic diagram of the AOTF based detector for
HPLC: PMT, photomultiplier tube; lock-in, lock-in amplifier;
RF, RF power amplifier; RF gen, RF signal generator.
photomultiplier tube. Its sensitivity is therefore
higher, and its cost is lower than the multichannel
detectors (i.e. DADs).
The schematic diagram of the AOTF based
detector is shown in Fig. 6. A 150 W xenon arc
lamp was used as the light source. Its output
radiation which contains UV, and visible light
was focussed onto the AOTF by a combination of
a reflector, collimator and lens. Acoustic waves
will be generated in the AOTF when the RF
signal is applied into the filter by a signal generator. To reduce noise and to facilitate the phase
sensitive detection the RF signal was sinusoidally
modulated at 50 kHz by the microcomputer
through the D/A. Prior to being connected to the
AOTF, the AM modulated RF signal was amplified to 5 W power by an RF power amplifier.
The intensity of the light diffracted from the
AOTF was detected by a photomultiplier tube
and demodulated (at 50 kHz) by a lock-in amplifier prior to being recorded by a microcomputer.
The chromatographic system consists of an isocratic pump and a sample injector valve equipped
with a 40 lal loop. A 250 m m x 4 . 6 mm I.D.
stainless-steel column packed with Nucleosil 5
silica was used. The chromatographic microflow
cell used in this study, which has 8 mm path
length and 8 ~tl volume, is similar to that used
previously [16].
Shown in Fig. 7 is the three dimensional graph
plotting the chromatogram as a function of time
and wavelength of a sample which was a mixture
of three phenol derivatives, i.e. 4-chloro-,
trichloro- and pentachlorophenol. The chromatogram as a function of time was obtained by
setting the AOTF at a single wavelength of 292
nm. Absorption spectrum of each compound was
measured as it eluted out of the column by rapidly
scanning the AOTF. Each single spectrum was
obtained by scanning the AOTF for 100 nm (from
250 to 350 nm) and recording 100 points (i.e. 1
point for each nanometer). The setting was selected so that it required 2 ms to record each
point. Therefore, the time required to record a
single spectrum is 200 ms, and it took 4 s to
obtained the spectrum, which is the average of 20
spectra for each compound. However, as evident
from the figures, only 60 nm (i.e. from 260 to 320
nm) is required to record the whole spectrum for
all three compounds. Therefore, with the optimal
setting of 300 Its time constant, 12 dB rolloff, and
2 ms pt J (on the lock-in amplifier), it requires
only 900 ms to obtain an average of 5 spectra
which has relatively good S/N.
0::
0.295
-
i
II
.04~
o~
Fig. 7. Three dimensional graph plotting the chromatogram of
mixture of pentachloro-, trichloro- and 4-chlorophenol as a
function of time and wavelength.
C.D. Tran / Talanta 45 (1997) 237-248
The calibration curve was constructed for
each compound over a concentration range of
1 . 0 x 10 - 4 2.0× 10 -3 M using the data obtained when the AOTF was fixed at a single
wavelength corresponding to the peak of the absorption spectrum of each compound, i.e. 285,
300 and 306 nm for 4-chloro-, trichloro- and
pentachlorophenol, respectively. As expected,
good linear relationship was obtained for all
three compounds (the correlation coefficients for
all three compounds were larger than 0.999).
The limits of detection (LODs) defined as twice
the peak-to-peak noise of the baseline divided by
the slope of the calibration graph, are estimated
to be 1.0x 10 -5 , 1.1 x l0 5 and 1.7x 10 5 M
for trichloro-, pentachloro- and 4-chlorophenol,
respectively. These LOD values correspond to
the mass defectivity of 59, 88 and 65 ng, respectively, and to the absorbance unit of 4.0 × 10 4
These detections limit are comparable with those
found on commercially available (grating or
filter based) single wavelength absorption detectors. Particularly, its detectability of 4.0 × 10 4
absorption unit is similar to the value of 3.9 ×
10 - 4 absorbance unit, which we have previously
determined for 4-chlorphenol using the Shimadzu model SPD-6AV VU-visible absorption
detector [16]. The LOD value of 4.0 x 10 -4 AU
is much smaller than those obtained using commercially available diode array detectors [28].
This is as expected because the present AOTF
based detector is a single channel detection technique, which is more sensitive than the multichannel detection employed in the diode array
detectors. Furthermore, the light source used in
this AOTF based detector is modulated at 50
kHz (through modulating the applied RF signal)
which facilitates the phase lock detection. The
S/N is further enhanced by this phase sensitive
detection. Other feature which makes this
AOTF-based detector more desirable than diode
array detectors is its high spectral resolution.
Specifically, the resolution of this AOTF-based
detector is 0.82 A at 253 nm [9]. This spectral
resolution is much smaller than those of the
DAD which are generally on the order of several nanometers [12].
245
3.2. AOTF based detectors for flow injection
analysis
Flow injection analysis (FIA) is among the
most widely used methods for automated analysis.
Its applications to several fields of chemistry has
been demonstrated in recent years [29,30]. Several
operational modes of FIA have been realized by
appropriately modifying traditional wet chemical
methods (dilution, extraction, titration, fast kinetic reactions) into automated flow devices
[13,14]. Different types of detectors, including
electrochemical, (UV and visible) spectrophotometric, and luminescent, have been applied to the
FIA. [29,30]. However, there has not been a FIA
detector which is truly universal.
As described in previous section, spectrochemical applications of the near infra-red absorption
technique (NIR) has increased significantly in recent years [15]. The popularity stems from advantages of the technique including the wide
applicability (all compounds that have C - H , O H and/or N - H groups have absorption in this
spectral region), the possibility of in situ applications (no need for sample pretreatment) and the
availability of powerful and effective multivariate
statistical methods for data analysis. These features enable the NIR to serve as a universal
detector for FIA. However, the detection of FIA
by NIR has not been fully exploited. In fact, in
there are only two reports describing the utilization of NIR for continuous-flow FIA detection
[31,32]. Unfortunately, in these studies, the potentials of the NIR technique have not been fully
exploited because they were based on the use of
only a single wavelength [31,32]. As a consequence, the multivariate calibration methods cannot be used to analyze data in these studies. This
drawback can be overcome by use of an AOTF
based NIR detector.
The construction of the AOTF based NIR detector for FIA, shown in Fig. 8, is essentially the
same as that of the AOTF based UV-visible detector for HPLC which was described above. The
only differences were those of the light source (a
100 W, 12 V halogen tungsten lamp), the AOTF
for the NIR region, and a the detector (InGaAs
photodiodes)
246
C.D. Tran / Talanta 45 (1997) 237-248
Lamp
--:-- Ph
-"
,~PO
L
AOTF
RF ampl ~ R F
gen
[
L
Ph --:--
w
W
4
InGaAs
S
Pump
I L°°ki° I
+
C
W
L
InGaAs
ILock- in
F
q
Computer
Fig. 8. Near-infra-red-FIA instrument: W, waste; C, carrier; Pump, peristaltic pump; S, sample; 1, injector; T, T connector; FC, flow
cell; Ph, pinhole; Po, polarizer; L, lens; BS, beamspliner.
This A O T F based detector covers a near IR
region from 1000 to 1600 nm. Because the combination and overtone absorption bands of O - H
and C - H groups are in this region, this FIAA O T F detector can be used for such determinations as water in chloroform, and water and
benzene in ethanol. For example, Fig. 9 shows the
F I A absorption peak profile (i.e. absorption spectrum as a function of time and wavelength), obtained when a solution of 0.10% (v/v) water was
injected into chloroform. It is evident that as the
concentration of water in the flow cell increases,
there is an increase in the absorption in the 13001500 nm region. The can be attributed to the first
overtone transition, and the combination of
stretching and bending of the O - H group at 1450
and 1180 nm, respectively [17]. The absorption
reaches its maximum 12 s after the injection and
then starts to decrease. Using the spectra measured 12 s after the injection for different concentrations of water a calibration model based on the
partial least square method (PLS) was developed
for the determination of water in chloroform.
G o o d correlation was obtained between the con-
0.030 I
I
!
/
!
0022 4
I]
8
0014
o
.o
<
0.006 4
I
5
e
!
I
]
0002
-o.olo ~ - ~ - ~ ~ k _ ~ ~ > ~ _
14601420--1380
n Orn
-~
j~
><y2O
1300
5
~\~e'5
25
Fig. 9. Absorption (flow cell using chloroform as blank) as a
function of time and wavelength after the injection of 0.10%
(v/v) water solution in chloroform.
C.D. Tran ,,; Talanta 45 (1997) 2 3 7 248
i
I
0.035
'
0.025
i
<
0.015
it). >';
i
,' : :',
[
"
;7//'~
'
0.005
-o.oo5
.....
"
-0.015
-
~
/s._~<,~:
'~
~:
1638
,<
1,141576
" ~ ."
/"
avel~ 1514 " - / _
~ngth- 1452 " ' ~
' '"tb
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4,...~,.~."
" 36
45
Fig. 10. Absorption (flow cell using ethanol as blank) as a
function of time and wavelength measured after the injection
of ethanol solution containing 0.6 and 1.5% (v/v) of water and
benzene, respectively.
centration of water injected and the concentration
of water calculated by the model (r = 0.99). The
RMSD for this determination was calculated to
be 0.002°/,,. The LOD at 1400 nm was found to be
15 ppm of water.
It is possible to sensitively and simultaneously
determine the concentrations of water and benzene in ethanol by NIR spectrometry. Fig. 10
shows the absorption measured as a function of
time and wavelength after the injection of sample
containing 0.6 and 1.5% of water and benzene
respectively. As can be seen from the figure, when
the sample is passing through the detector between 8 and 26 s there is an increase in the
absorption in the 1650-1700 nm and 1390 1500
nm regions (due to absorption of benzene and
water, respectively) and a decrease in the 15001650 nm region (due to benzene). These spectral
profiles are the same to those observed in the
previous measurements using the non-flowing 1
cm cuvette (figure not shown). In order to produce a calibration model for the simultaneous
determination of benzene and water in ethanol
using NIR-FIA technique, 21 samples containing
different concentrations of water and benzene
were prepared. Each sample was injected four
times. Good correlation was obtained between the
concentration of water and benzene injected and
247
the concentration of both components calculated
by the PLS model (SR = 1450-1496 nm, 16721700 nm, 1583-1602 nm; N F = 4 and 3 for water
and benzene respectively). The statistical parameters obtained were r = 0.997 and R M S D = 0.015%
and r = 0.997 and R M S D = 0.033% for water and
benzene respectively.
In summary, it has been demonstrated that due
to its high scanning velocity and wavelength accuracy the A O T F based detector for FIA can measure whole NIR spectra of flowing samples within
the time frame required for flow analysis. This
allowed the utilization of multivariate statistical
methods of analysis, which in turn, increase the
sensitivity, accuracy and applicability of the technique. In fact it was possible to perform not only
a simple analysis, such as the determination of
dryness of organic solvent (i.e. the concentration
of water in chloroform) but also a more complex
analysis including the simultaneous determination
of two component systems (i.e. the concentration
of water and benzene in ethanol). For simple one
component systems, the LOD of this AOTF-NIRFIA technique is comparable to that of the FIA
spectrophotometry as well as to that of the single
wavelength NIR-FIA technique. In this case the
advantages of the A O T F - N I R - F I A instrumentation are its sensitivity, automation and wide applicability.
For relatively more complex systems (e.g. two
component systems) the advantages become more
prevalent. In fact, there is not a method currently
available that can be easily coupled with FIA
instrument for the sensitive and simultaneous determination of the concentrations of two or more
components without any sample pretreatment.
The present automated and real-time determination of water and benzene in ethanol is important
because ethanol is increasingly being used as a
substitute and/or additive to gasoline (i.e. it is
important to know the concentrations of water
and benzene impurities in such systems).
Acknowledgements
The author wishes to thank his present and
former coworkers, whose work is cited in the
248
C.D. Tran / Talanta 45 (1997) 237 248
references. A c k n o w l e d g m e n t is m a d e to the N a t i o n a l I n s t i t u t e s o f H e a l t h for f i n a n c i a l s u p p o r t o f
this w o r k .
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